Pedestrians pass the arch in Washington Square park Thursday in New York. A man accused of keeping bomb-making materials in his Greenwich Village apartment told acquaintances that he planned to blow up the Washington Square park arch police said Thursday.

NEW YORK — A man accused of keeping bomb-making materials in his Greenwich Village apartment told acquaintances late last year that he planned to blow up the Washington Square Park arch, police said Thursday.

The acquaintances have also informed police that in another encounter with Aaron Greene, they saw him sprinkle white powder on the sidewalk inside the park and ignite it, said Paul Browne, chief spokesman for the New York Police Department.

Investigators believe the powder was hexamethylene triperoxide diamine, or HMTD, a highly explosive ingredient used in bomb making that was discovered last month in the apartment Greene shared with his girlfriend, Morgan Gliedman, Browne said.

Though Greene’s motives remain unclear, investigators have uncovered undated letters that he wrote, including one with the words “kill them all” and signed with a Nazi SS lightning bolt symbol still used by neo-Nazi groups, police said. Other witnesses also have told police that they heard him boast that his grandfather was a Nazi, they added.

There was no immediate response to a message left Thursday with an attorney for Greene.

Greene, 31, and Gliedman, 27, were arrested last month after a police search of their home turned up papers titled “The Terrorist Encyclopedia” and “Deadly Homemade Weapons,” along with 7 grams of HMTD. In the living room, there also was a sawed-off shotgun, a commercial replica of a grenade launcher and ammunition, and chemical precursors to HMTD, police said.

Police have since learned that there “was some discussion about the possibility of blowing up” the large arch on the north side of the park that honors the first U.S. president, Police Commissioner Raymond Kelly said Thursday.

Investigators “should always take a statement like that seriously, particularly when in that person’s apartment, there were manuals that talked about explosives and demolition,” Kelly added.

Based on an interview with Greene, police searched the Orangeburg, N.Y., home of one of his childhood friends, a suspended jail guard, on Wednesday, Browne said. Officers recovered several weapons, including nearly two dozen guns and a switchblade, he said.

Gliedman, who was nine months pregnant at the time of her arrest, went into labor while in custody and gave birth to a daughter, Melanie, on Dec. 31. She was released on bail on Wednesday and ordered to complete a 30-day drug rehab program.

Greene is being held without bail. Both are due back in court on Jan. 29.